The subject matter disclosed herein relates generally to bleed air systems for gas turbine engines, and, more specifically, to bleed air systems capable of providing pressure drops, acoustic improvements, and desirable flow properties.
In a gas turbine engine, air is pressurized in a compression module. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases, which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors and generate engine thrust to propel an aircraft in flight or to power a load, such as an electrical generator.
In some gas turbine engines, a portion of the high-pressure air, such as, for example, bleed air from a compressor, may be extracted or bled from the compressor for various needs. These needs include, for example, compressor flow bleeding which may be used to improve operability as well as to provide turbine cooling, bearing sump pressurization, purge air, or aircraft environment control. The air may be bled off from the compressor using bleed slots located over specific portions or stages of the compressor.
In at least some gas turbine engines, during engine operation occurring in some operating conditions, the compressor may pump more air than is required for needs including the combustion process. In order to manage operability of the engine and combustion performance, a portion of the excess bleed air from the compressor may be routed through bleed conduits and exhausted into the bypass flow stream, engine exhaust, or to ambient. The pressure and temperature of the air stream bled from the compressor may be very high. For example, bleed air pressure may be greater than about 1375 kPa and the bleed air temperature may be greater than about 538 degrees C. A transient bleed valve system (TBV) system and/or a variable bleed valve (VBV) system is sometimes used for bleeding and exhausting the air removed from the compressor. For example, the exhaust area of some conventional bleed systems may be oversized to lower the flow velocity at the exhaust location to assure that the acoustic requirements are met for the application. The exhaust area, as well as the expansions between the source pressure and exhaust, may contribute to the large size and/or weight of these systems.
In addition, some exhaust designs on aircraft may require extensive thermal shielding on other components near the exhaust location. Due to the nature of the high temperature and high pressure air, once it is discharged into the flow path, it may overwhelm the flowpath stream, causing the bleed air to impinge on the surrounding structure around the engine. In some aircraft, the surrounding structure may be made of lightweight composite material or of other metallic material with lesser temperature capability.
The problem: In some applications (e.g., aircraft), it may be necessary to direct bleed air flow into the bypass stream to avoid thermal damage to bypass duct components.